consiletter 4

Identifying very low vapor pressure (e. g. of solids)

No data, no market: That’s what it says in Article 5 of the General obligation to register and inform requirements for chemicals covered by the new REACH Regulation, which are currently being introduced incrementally. Because of this, determination of vapor pressure will soon play a larger role in the chemical industry. Based on REACH, determining vapor pressure falls under the standard requirements for physiochemical studies (see REACH Regulation Annex VII 7.5) and is therefore required for each registration of a substance.

effusion method

Vapor pressure can be determined using the various methods described in the OECD Guideline 104. With the effusion method/vapor pressure balance, vapor pressure can be determined from solid and liquid substances in a pressure range from 10^-3 to 1 Pa (or 10^-5 – 10^-2 mbar). This range is especially important for medium- to low-volatile substances.

The principle of the effusion method is based on a molecular beam that is emitted through the defined opening of a Knudsen effusion cell and that condenses on a target plate cooled with liquid nitrogen. The vapor pressure of the test substance can be computed with the Knudsen effusion equation from the increase in weight of the target plate, which is measured using a highly sensitive microbalance (see Figure 1). In Figure 2, the vapor pressures of benzoic acid measured at various temperatures are plotted over the inverse temperature. The Antoine parameters can be determined from the straight lines formed in the semi-logarithmic diagram, with which the vapor pressure at other temperatures can be extrapolated. The unique aspect here is that substances with a very low vapor pressure (e. g. solids) can be reliably measured this way.

In addition to the described vapor pressure balance, the consilab laboratory features a second device that determines vapor pressure from the loss of mass caused by the effusion (recommended measuring range according to OECD 104: 10^-10 to 1 Pa).

As a certified GLP testing facility, consilab naturally also offers determination of vapor pressure in the form of a GLP study. In addition, our laboratory is equipped with all other physiochemical methods required for the new approval of substances.

If we can assist you with an issue related to determining vapor pressure, please contact us. Our experts will be happy to help you.

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consiletter 3

Designing Safety Valves for the fire case?

Frequently, the question arises whether safety valves on containers should also be designed for the case of fire. In TRB 403 of October 1998 „Equipment of Pressure Vessels – Devices for recognizing and delimitating pressure and temperature“ the response to this question still was a definite NO. Here, paragraph 3.2.3 says: „When designing safety facilities, it is NOT necessary to consider the fire case.“ In the subsequent guidelines, AD 2000 Information Sheet A 403 with the same title, paragraph 3.2.3 the June edition 2001, however, contains the following statement: „Taking the external fire case into consideration MAY be necessary when designing safety devices against overpressure.“ Meaning: Today immediate answers to this question can no longer be given with absolute certainty.

In API 521 or ISO 23251, the process of calculating heat input for the fire case for different containers under varying surrounding influences is described in detail. Here, it is not only deemed important whether the container is equipped with a heating jacket but also, how quickly fire fighters can be on the site, or, whether there is a sprinkler system. Figure 1 shows the heat input depending on the filling level of a 10 m³ container filled with acetone being heated with differing steam temperatures. For the sake of comparison, the heat input due to fire as calculated by normative formulas is also displayed.

In the most sites of chemical plants, as is also the case in Industrial Park Hoechst, fire fighters arrive on scene in less than ten minutes. Thus, cases in which the worst case scenario need be assumed are indeed rare. The fact that, by standard, many plants possess sprinkler systems, significantly even more reduces heat input due to fire in most cases. As the diagram shows, the design case of „External Heating “ at 6 bar steam normally yields much more heat input than the fire case.

This means, that often, standard containers with heating jacket need not be designed for the fire case if external heating by steam has already been considered. However, such consideration must be given in the case that the heating jacket is designed to function as isolation in the event of underfiring. Safety briefings for containers without heating jackets thus should take special care to ensure that the case of underfiring is effectively excluded, for example, by designing drain channels to prevent accumulation of inflammable/combustible liquids underneath the container.

If you have any questions regarding similar cases, please do not hesitate to contact us. Our experts are always very happy to assist you.

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consiletter 2

Determining the Self-Ignition Temperature of Dust Deposits

Most solids produced or handled in the chemical industry are powders. These substances have a large surface/volume ratio enabling oxidation processes to occur already at room temperature. Due to the poor heat dissipation of bulk materials, the substance might heat up or even ignite on its own. According to the relevant standards and regulations “self-ignition” is defined as a temperature increase by more than 60 K (compared to the initial storage temperature). If the temperature increase is less, this is denominated “self-heating”.

Generally, the self-ignition temperature is determined by several isoperibolic heat storage tests at different constant temperatures and different sample sizes. This method takes a lot of time and needs large amounts of the substance. However, our customers frequently can only provide a relative small amount of the material for the required investigations.

To gain an economy of time, costs and material for our customers, we offer the advantages of adiabatic wire basket tests. For this purpose the sample is filled into a wire basket of a volume of 400 ml for isoperibolic storage over 24 h. If no self-heating occurs within this period of time, the oven temperature is increased so slowly, that a uniform temperature distribution in the sample volume can be assumed. As soon as the sample temperature exceeds the oven temperature, the oven temperature is regulated to trace the sample temperature. Thus, adiabatic conditions are provided in this phase of the test.

Figure 1 shows the experimental set up as used in our laboratory. Additionally to the core temperature, the temperature at the surface, and the temperature midway from center to surface are measured.

A typical kinetically evaluated test is depicted in Figure 2. The sample was tempered at 50°C and subsequently stored at 100°C for 24 h. As no self-heating occurred, the oven temperature was increased by 1 K/h. At approximately 152°C the sample temperature exceeded the oven temperature and self ignition came to pass.

This kind of experimental procedure induces advantages for our customers. Due to the adiabatic conditions the values for the activation energy EA, the Arrhenius prefactor k0 (from the linearized slope of the temperature rise curve caused by self-ignition), and the thermal conductivity a can be derived from the tests. From these values plus the geometry of the sample, the required self-ignition temperature can be evaluated iteratively using the “theory of stationary heat explosion” by D. A. Frank-Kamenetskii. In contrast to isoperibolic experiments, the self-ignition temperature for larger bulk sizes can be directly derived from the measured values with respect to the geometry. An extrapolation procedure is not required, and the activation energy is considered. The obtained temperatures are significantly more reliable, which leads to a considerably higher safety level for our customers.

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consiletter 1

Safeguarding of a supply line for pressurized air

As in many of our customers’ laboratories, the apparatuses in the laboratories of consilab have to be supplied with technical gases, too. In most of the cases, the tasks of pressure vessel safety are similar and the questions recur. For this reason, the assembly in the consilab laboratory shall be described as an example.

Commonly, compressed gas cylinders can only be operated when equipped with a pressure reducing device. But what is going to happen, if the pressure reducing device of a new, completely filled gas cylinder is damaged or fully opened when connected to the laboratory equipment? The total pressure of the cylinder could propagate via open pipelines into the apparatuses and eventually destroy those. To prevent this, a safety valve is to be installed into the feed line, to set the maximum pressure of the system to the maximum allowable pressure of the connected apparatuses.

The sizing of safety valves is part of the range of services offered by consilab. First of all, the mass flow rate to be discharged has to be calculated. This is the mass flow rate of gas which can flow from the gas cylinders into the connected apparatuses. Basis of the calculations are the pressure difference between maximum pressure in the gas cylinder and the set pressure of the safety valve or the maximum allowable working pressure of the apparatus. The geometric assembly of the feed pipe (diameter, bends, length of pipe from pressure source to the place where the safety valve is connected) can be taken into account for the calculation. In many cases the seat area of the pressure regulator, which could open completely due to a full break of the membrane, is most relevant for the result.

In the next step a safety valve has to be chosen. The dischargeable mass flow at set pressure has to be equal or higher than the mass flow to be discharged which was calculated in the first step. On the other hand, the safety valve should be as small as possible, since the gas feed lines are usually very small and there is not much space in a fume hood. But not every safety valve is applicable to reliably venting the required gas mass flow.

Because of the high pressure difference of the compressed gas cylinder (300 bar at consilab) and the apparatus (30 bar at consilab) and the relatively low pressure difference between set pressure and ambient pressure, the required venting area is accordingly high.

In the actual case a safety valve by Leser, size ½”/ ½”, type 437, is sufficient to protect the feed line in size DN6. The easiest way to connect the safety valve to the feed line seems to be to insert a branch tee and expand the line behind it to the size of the safety valve inlet. However this is not permissible. Standards like ISO 4126 regulate that there must not be a diameter smaller than the safety valve inlet diameter in the line between the room to be protected and the safety valve itself. Additionally, the pressure losses in the line to the safety valve are to be restricted to 3% of the set pressure. To meet these criteria, the diameter of the feed line has to be expanded to the safety valves inlet size before the branch tee. An example of an ideal assembly for this task is shown in the photo.

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